This invention relates to a method of processing ores containing one or more metal values selected from the group consisting of copper, nickel, cobalt, vanadium, and manganese which are present in the ore in association with iron. The process rapidly and quantitatively separates the non-ferrous metals from the iron. More particularly, the invention relates to an improvement in the process of selective sulfation of such ores to render the non-ferrous metals leachable and the ferrous metals non-leachable.
It is well known that ores containing pentlandite, pyrrhotite, chalcopyrite, cubanite, other sulfides of copper, other sulfates of nickel, small amounts of cobalt, vanadium and manganese, and various mixtures thereof may be treated to yield metal values by selective sulfation. In general, this process involves oxygen roasting the concentrates in the presence of sulfur dioxide or a sulfate salt to produce sulfates from the copper, nickel, and/or non-ferrous metals present and to convert the ferrous metal values to Fe.sub.2 O.sub.3. Since the sulfates of the non-ferrous values are soluble, after such treatment, it is possible to water leach the mixture, leaving the ferrous values behind and conveniently extracting the non-ferrous values.
While this process has considerable theoretical potential, it has achieved only limited success in large scale commercial use because of several difficulties which make it economically unfeasible. The most basic disadvantage of the sulfation process is that the temperature and the roaster gas composition are difficult to control so that the production of sulfates of the non-ferrous values is maximized. Although the reaction of iron to form iron oxide generally requires a temperature in excess of about 675.degree. C, at such temperatures much of the copper, nickel, etc. may also be oxidized to a non-leachable state. Below about 675.degree. C, much of the iron is converted to sulfate. Unless a high degree of partitioning between the ferrous and non-ferrous metals can be achieved, the water leach extracts an unacceptably high percentage of the ferrous metal values along with the metal values of interest.
The prior art is replete with attempts to improve the yield of water soluble copper values in the sulfation process. For example, U.S. Pat. No. 3,441,403 to R. E. Fredrickson et al. teaches that the yield of soluble copper may be increased by adding gaseous HCl to the roaster gas. In U.S. Pat. No. 2,719,082 to W. K. Sproule et al., a method for producing high grade hematite from nickel, copper, and cobalt containing iron sulfide ores is disclosed which takes advantage of this sulfation process to remove small amounts of non-ferrous metal sulfide contaminates and small amounts of silica, lime, alumina, and magnesia from the ore. This process involves removing the bulk of the contaminates by conventional separation techniques and then roasting the concentrated and partially purified ore under oxidizing conditions to form a permeable iron oxide calcine containing not more than 1% nickel and 0.1% copper. The calcine is then sulfated by heating it between 630.degree.-687.degree. C (1200.degree.-1300.degree. F) with between 2% and 8% by weight sodium sulfate in the presence of a roasting gas comprising between 4 and 6% sulfur dioxide and more than 5% oxygen. This process is said to be capable of rendering "substantially all" of the small amount of copper present and up to 86% of the nickel present water soluble. However, it is taught that these results can only be achieved when the ore has been pretreated to contain the small recited percentages of copper and nickel.
U.S. Pat. No. 3,791,812 to R. L. Frank et al. discloses a process for extracting copper, cobalt, and manganese values from ores as water soluble salts. In this process, a sulfide ore bearing the metal values of interest is mixed with an inorganic chloride to form a mixture containing from about 30-93 weight percent ore and from about 7 to about 70 weight percent inorganic chloride. A charge of the mixture in a gas permeable state is roasted with oxygen at a temperature of about 300.degree. to 425.degree. C and the sulfur dioxide produced is transferred to a second stage roasting zone where the oxides produced in the first zone are converted to sulfates. The primary object of the invention disclosed in this patent is to reduce sulfur dioxide emissions, and no data is presented which indicates that the ferrous metals and non-ferrous metals are efficiently partitioned. In fact, the temperature range disclosed in this patent indicates that a substantial amount of soluble iron sulfate and iron chloride would be produced by the process and that a substantial amount of iron would therefore be leached along with the non-ferrous metals.
In general, it is believed that all the prior art sulfation processes designed to render substantial amounts of non-ferrous metal values in extractable form, yield a non-ferrous metal sulfate which is contaminated by unsuitably large quantities of iron sulfate and generally require a roasting time which is greater than optimal. In contrast, embodiments of the process of this invention yield up to 99% of the copper, 97% of the nickel, 90% of the cobalt, and comparable percentages of other non-ferrous metals while excluding between 96% and 99% of the iron in a short period of time and under conditions of temperature and pressure which are easily regulated.